1. Field of the Invention
The present invention relates to an image forming apparatus and a powder pump utilized therein, and more particularly to an image forming apparatus that uses electrophotography to form an image with a toner which is conveyed by a screw pump and fed by gas (mixed gas) across a distant path from a developing station towards a latent image formed on an image bearing member thereof.
2. Discussion of the Background
In image forming apparatuses such as printers, facsimile machines, copying machines, and multi functional machines, toner is transferred onto a photoconductive element for forming a toner image. The toner image is transferred onto a transfer sheet and used residual toner that has not transferred onto the transfer sheet is collected by a cleaning device. The used residual toner is recycled to be reused for developing during a subsequent image forming operation by returning the residual toner to the developing device.
It is well known that the toner in the developer is supplied to the developing device by a pump, circulating with flowing gas.
As shown in FIGS. 12 and 13, a system is shown for reusing a residual toner which has not adhered onto an image bearing member 101 employed as a drum-shaped photoconductor, or onto a belt-shaped transfer member (not shown). The residual toner is scraped off of image bearing member 101 by a cleaning blade 107a and collected in cleaning unit 107. The collected toner is exhausted from an exhaust pipe 107b of the cleaning unit 107 and is dropped into a lateral conveying screw case 108a via a connecting device 108. A rotational drive power of a drive motor 109 is transmitted to lateral conveying screw 108b in the lateral conveying screw case 108a, via a belt 109b and a pulley 109c forming a rotational drive power transmitting device 109a. The collected residual toner is conveyed into a stator 103a by rotation of the lateral conveying screw 108b. Lateral conveying screw 108b rotates together with a rotor 103b in the stator 103a, stator 103a being held in position by a holder 103c of a screw pump 103.
The toner is conveyed by rotation of the rotor 103b in the stator 103a of the screw pump 103 into a mixed gas conveying device 105 that is connected to an exit 103d of the stator 10a. A pump 104 blows air into holder 103c at the same time. The air is collected from a developing device 102 and is circulated via conveying pipe 104a, into the screw pump 103 through the air conveying tube 104a, and the air mixes with toner in the mixed gas conveying device 105. Toner scattering which can occur when decompression occurs at a hopper 102a of the developing device 102 is prevented since an air suction pipe of the powder pump 104 is connected to the hopper 102a of the developing device 102 and the gas is conveyed from the mixed gas conveying device 105 to the hopper 102a of the developing device 102. Further, there has been proposed by the inventor of the present invention an image forming apparatus in which toner is supplied from a position distant from the developing station, to the developing device 102 and having an air circulation path with little air loss.
However, such a conventional image forming apparatus has the following shortcoming. Each element of the apparatus is positioned to form a circulation path for circulating the air for fluidifying the toner passed by the mixed gas conveying device 105. The elements are arranged in the order of the hopper 102a for supplying a toner to the developing device 102, the powder pump 104, the screw pump 103, and the hopper 102a for supplying the toner to the developing device 102. However, over time, a gap tends to form between the stator 103a and the rotor 103b of the screw pump 103 which are respectively composed of rubber materials. The gap is formed because of normal wear of the stator 103a and rotor 103b over time caused by contact and engagement between the stator 103a and the rotor 103b. Therefore, air inside of the screw pump 103 gradually leaks out to the open air, and consequently the toner conveying ability from an exit 103d of the screw pump 103 gradually decreases Eventually, the toner cannot be supplied to the hopper 102a for supply to the developing device 102 and an image quality deteriorates. (Refer to FIG. 13)
Furthermore, in such a conventional image forming apparatus, if the powder pump 104 is located at a position lower than the developing device 102 in a vertical direction for reasons such as, for example, saving space, simplifying the drive power source mechanism by using the same drive source for both the powder pump 104 and the other units, or the like, the following shortcomings may occur. The powder pump 104 may malfunction, i.e., the outlet valve (not shown) of powder pump 104 may lock in a closed position due to a build up of toner, or the outlet valve may not be able to fully close shut, or the like. This can occur because toner can get stopped up at the outlet valve side of the powder pump 104 because of a flowing-back of the toner from conveying device 105 or screw pump 103 and/or tube 104a due to vibration and gravity effects, for example, since toner will typically remain on an internal wall of mixed gas conveying device 105, pump 103 and/or tube 104a after the powder pump 104 has stopped.
A powder pump is often used to feed toner powder, or two-component type developers composed of a toner and carrier, which are used in an image forming apparatus such as a copying machine, a printer, a facsimile device or the like. Such a powder pump, typically includes a stator having two spiral grooves formed at an internal peripheral surface thereof that is composed of an elastic material, and a rotor disposed in the stator. This powder pump is called a one-shaft eccentric screw pump. The screw pump exhales powder (fed from an inlet) from an outlet thereof by rotational drive of the rotor. In such a powder pump, air is fed with compression from a secondary powder pump in order to increase a fluidity of the powder that is exhaled from the powder pump. Thus, in the case of using the powder pump of this type, there is required a secondary powder pump for feeding air to the powder pump resulting in a large-sized structure, and increased manufacturing costs.
Further, if the powder pump is used for extended periods of time, the material of the stator deteriorates and the stator is permanently deformed since the stator is composed of an elastic material such as rubber or the like. Accordingly, air-tightness of a space between the stator and the rotor cannot be maintained, and conveying ability of the powder deteriorates, and ultimately, powder conveyance becomes impossible.
On the other hand, a pump is also well known which draws a fluid into a chamber that is partitioned with a case and a diaphragm from an inlet that is formed on the case, and then ejects the fluid in the chamber from an outlet. The inlet and the outlet of the pump open and shut by operation of an inlet valve and an outlet valve respectively, that are operated by up and down movement of the diaphragm and corresponding pressure changes in the chamber. The pump of this type has a relatively small structure and is manufactured at low cost in comparison with the aforementioned powder pump, having an advantage in this point.
FIG. 11 is a cross sectional view showing such a conventional pump in which a diaphragm is used to convey fluids. Fluid can be conveyed by opening/shutting the inlet 110 and the outlet 106 with movement of the inlet valve 130 and the outlet valve 140 by operation of the diaphragm 5. However, if the pump 100 is used for conveying a powder, the powder tends to accumulate around the powder inlet 110 and the powder outlet 106, or on the inlet valve 130 and the outlet valve 140. Thereby, when the inlet valve 130 and the outlet valve 140 are shut, the valves cannot make close contact at the surfaces of the case wall around the inlet 110 and the outlet 106. That is, air-tightness of the inlet valve 130 and the outlet valve 140 is lost. In this state, a problem occurs that, when the pressure in the working chamber 8 is increased, the powder in the working chamber 108 is discharged through the inlet 110, and when the pressure in the working chamber 108 is decreased, the powder flows into the working chamber 108 through the outlet 106. Therefore, the conveying ability of the pump when used to convey powder is deteriorated. Thus, even though the conventional pump shown in FIG. 11 has an advantage that the size of the pump is more compact than the one-shaft eccentric screw type pump and the manufacturing cost is relatively low, the conventional pump cannot be used for conveying powder.
In other words, even though this conventional pump having the diaphragm can often be used for conveying gas and liquid without problem, it is not without its drawbacks. Due to the configuration of the pump housing sections 102 and 103, if the pump is used to pump powder, the powder would tend to accumulate around the inlet and the outlet that are to be shut by the inlet valve 130 and the outlet valve 140, respectively, or around the inlet valve 130 and the outlet valve 140 themselves. Accordingly, the inlet and the outlet would not be able to be shut securely with the inlet valve 130 and the outlet valve 140, resulting in deterioration of efficient conveyance of the powder. Thus, although the pump of this type is advantageously used for conveying gas and liquid, this pump cannot conventionally be used for conveying powder.
In light of the above considerations, it is highly desirable to provide a pump having a diaphragm (hereinafter called powder pump) which can be used for conveying powder and having structure of compact size and of low manufacturing costs which is stable for use over long periods of time. Further, it is also highly desirable to provide an image forming apparatus utilizing the powder pump for conveying toner in which toner which remains on an internal wall of an elastic tube or the like is prevented from flowing-back to a powder pump side even when vibration occurs to the apparatus after the powder pump is stopped, and thereby the powder pump can be operated under normal conditions resulting in high image quality performance.